1. Field of the Invention
The present invention relates to a differential signal peak detector that eliminates the common mode component, thereby generating an accurate peak measurement.
2. Description of the Related Art
Radio frequency (RF) integrated circuits (ICs) are increasingly ubiquitous in the telecommunications industry. In such an RF IC, a peak detector is often used to generate a low frequency signal proportional to the amplitude of an input RF signal. Knowing the amplitude, other circuits on the RF IC can be adjusted to improve signal detection and analysis.
FIG. 1 illustrates a simple peak detector 100 including a diode 101 receiving an RF signal on its anode, a capacitor 102 coupled to the cathode of diode 101, and a resistor 103 also coupled to the cathode of diode 101 and connected in parallel to capacitor 102. In this configuration, the peaks of the RF signal can charge up capacitor 102, wherein resistor 103 can bleed off some of that charge, particularly during off peak times. Peak detector 100 operates by detecting a time varying input voltage, i.e. the RF signal, and storing charge on capacitor 102 to produce a voltage DET equal to the maximum detected input voltage.
Increasingly, RF ICs implement mixed signal designs that include both digital and analog circuits. Typically, RF IC analog circuits are differential, i.e. signals with equal magnitude and opposite phase are transmitted and processed. Using differential techniques throughout the RF IC can provide several advantages. For example, input RF signals can be made differential to increase their amplitude relative to a limited supply voltage, thereby allowing easier detection. This differential RF signal can also advantageously provide immunity to single-ended noise sources (e.g. supply noise).
Unfortunately, RF IC designers often use a single-ended peak detector, e.g. peak detector 100, for a differential RF signal and mistakenly assume that a common mode component (i.e. the sum of the differential signals divided by two) is negligible. Specifically, the RF signal frequently includes a common mode component that varies. In this case, peak detector 100 would follow the peak of that RF signal including the common mode component, thereby resulting in a varying and inaccurate peak measurement. The resulting error in the detected signal DET can undesirably decrease performance of the peak detector as well as any circuit (e.g. amplifiers) using the detected signal as an input.
Therefore, a need arises for a peak detector capable of sensing the peaks of a differential signal but rejecting its common mode component.